1. Field of the Invention
The invention relates to methods of curing coatings on lenses. More specifically, the invention relates to methods of curing one or more coatings on one side of a lens without damaging an existing coating or coatings on another side of the lens.
2. Description of Related Art
Lenses suited for use in eyeglasses, such as ophthalmic lens, typically have one or more coatings on either or both sides of the lenses. Such coatings include hard coatings, which give the side of the lens to which they are applied abrasion-resistant properties; anti-reflective coatings, which help eliminate reflections and, consequentially, reduce the fatigue and/or stress users may experience while driving at night or working in front of a computer; and mirror coatings, which give the lens a shiny, mirror-like look and tend to reflect harsh light rays emitted from surfaces such as snow and water. These coatings may be applied to a lens at different times during the manufacturing process.
In some cases, the laboratory of the business where a customer goes to have his eyeglass prescription filled will apply one or more of the coatings to the lens. In some of these cases, the lens will already have one or more coatings applied to its front, or convex side, when it is shipped to the laboratory/business. Specifically, the lens may have a hard coating, an anti-reflective coating, or both coatings on its front side. As another example, the lens may have a hard coating, a mirror coating, or both coatings on its front side.
Depending on the customer's preference, the laboratory may then apply a hard coating, an anti-reflective coating, or both, to the back, or concave, side of the lens. The laboratory may also apply a hard coating, an anti-reflective coating, or both, to the front side of the lens if no such coating(s) exists or if one or more such coatings are needed. The same is true for a hard coating and a mirror coating (as opposed to an anti-reflective coating). Prior to applying coatings to a lens, a laboratory will typically surface the lens to the appropriate prescription power, according to the customer's needs.
The coatings that may be applied to either or both sides of a lens may be UV-curable. “UV-curable” means curable through the application of ultra violet rays to the coating. The coatings that may be applied to either or both sides of the lens may alternatively be heat-curable.
Different coatings respond differently to heat. Hard coatings tend to be damaged through cracks or cracking if heated too much. Both anti-reflective and mirror coatings are more sensitive to heat than hard coatings. Anti-reflective and mirror coatings also tend to crack if heated too much. Typically, however, damage to anti-reflective and mirror coatings is described as crazing, which means becoming covered with fine cracks. Anti-reflective and mirror coatings tend to craze when heated too much because they do not possess a good ability to expand under the influence of heat. A hard coating is typically at least 20 to 50 times thicker than either anti-reflective or mirror coatings.
It is known to apply a UV-curable hard coating to the back side of a lens, and to cure that UV-curable hard coating without damaging a hard coating, an anti-reflective coating, or both, that exist on the front side of the lens.
It is also known to cure a thermally-curable hard coating on the back side of a lens using a thermal curing fluid from 3M, without damaging a hard coating on the front side of the lens. The lens was dipped into the thermal curing fluid, which had a temperature of around 132° C. The lens was made of polycarbonate (i.e., it was a polycarbonate lens as that term is defined below), and the front side hard coating was mostly acrylic or soft organosilicone. The front side did not have an anti-reflective or mirror coating.
It is also known to bake a lens having a thermally-curable hard coating deposited on a UV-curable primer on the back side of the lens without damaging a hard coating that existed on the front side of the lens. The lens was made of polycarbonate. The front side did not have an anti-reflective or mirror coating.